Add support for the nginx X-Accel-Redirect header

[samm@…]

Hi,

I am using X-Accel-Redirect with Trac in FastCGI mode by changing header name in the api.py. I am recommending to make this header name configurable or to add use_x_accel_redirect variable in the config. I can provide patch for this change. On the nginx side appropriate location looks like that:

    # serve attachments  by nginx directly
    location /data/trac/files/attachments/ {
        internal;
        alias /data/trac/files/attachments/;
    }